Project Summary/Abstract Natural killer cells (NK) are the innate counterpart to CD8 T cells, endowed with the ability to rapidly kill virus-infected cells and cancer cells and produce proinflammatory cytokines (IFN?, TNF). These functional attributes make NK cells an attractive target for immunotherapy against multiple types of cancer, and they have been proven useful in graft versus leukemia. However, appropriate regulation of NK cell function is needed to avoid autoimmunity, immune deficiency, and NK cell transformation. Therefore, understanding how NK cell number and function are regulated is essential for the effective use of NK cells as therapeutic agents for eradication of disease. The mature NK cell population is composed of multiple phenotypically distinct subsets that have unique functional abilities. A subset of mature NK cells (mNK1), with a CD27+CD11b- phenotype, maintains the cytotoxic effector pool (CD27-CD11b+) as well as generating ?memory? NK cells. Our work seeks to understand the transcriptional basis for maintenance of the mNK1 population and the mechanisms controlling their differentiation and cytokine responsiveness. In this grant we will test the hypothesis that E protein transcription factors induce a transcriptional program that maintains the CD27+CD11b- precursor population and that this program is ID2 during effector maturation. We will also test the hypothesis that the transcription factor ETS1 collaborates with E proteins and E protein targets to control mNK1 differentiation and cytokine induced activation. This work will provide a foundation for understanding the mechanisms that control NK cell number and function and provide insight into the transcriptional network that can be manipulated to control the function of these cells in normal and diseased states.